Polyphase filter circuit



Jan. 5, 1932. R D. DUNCAN,` JR 1,839,301

POLYPHASE FILTER CIRCUITS K v Filed May 7, 1927 2 Sheets-Sheet l INVENTOR.

BY di A TTORNY Jan. 5, 1932.

R. D. DUNCAN, JR

POLYPHASE FILTER CIRCUITS 2 sheets-sheet 2 Filed May Y. 192'7 INVEN TOR.

ATTORNEY Patented Jan. 5, 1932 tric@u PATENT ROBERT D. DUNCAN.'v JR., or EAST CRANCE, NEW JERSEY, ASSIGNOR, .BY MESNE ASSIGNMENTS, CALIFORNIA TO FEDERAL TEL-EGRAPH COMPANY, A OORPORAJIION OE BCLYPHASEFILTER CIRCUIT Application filed May 7,

My invention relates broadly to wired radio broadcasting systems and more particularly.

to polyphase high frequency systems of wired radio broadcasting.

y One of the objects 4of my invention is to provide a filter circuit arrangement for thev polyphase currents which are superimposed ona wirenet work for the broadcasting lof signaling energy. y V i" A .another object of my invention is to vprovide a three phase filter system or Wired radio broadcasting circuits.v l l n l have described my invention Ain the following specification by reference to the accoinpanying drawings in which Figure l shows a typical circuit arrangement for a polyphase wired radio transmis-v sion system in which the circuits of my invention are employed;

' Fig. 2 shows a modified arrangement of polyphase transmission circuit-s; y

Fig. 3 illustrates the application ofthe polyphase filtercircuit of my invention toa polvphase transmission system; 1 Fig. l shows the connection of a polyphase filter system according to my invention in an open delta circuit and shows a three wire two phase system employing the filter circuits of my invention.

' A band frequency filter' consists of a successionof inductive and Capacity elements so interconnected. thatfwhen interposed between a source of alternating current and the load in which energy is to be expended7 it will highly attenuate 'currents ofv certain frequencies but will pass with practically Zero attenuation cnrients of other frequencies.

-T he use of filters in the fields of telephony n and telegraphy is well establishedfthis is 40 also true to a limited extent for the case of high frequency in both space and wired radio where singlephase alternating currents .are

dealt with. However, in wired radio broadcasting. oi' in general where `polyphase high 4 frequency currents are employed the use of filters'presents a number of new problems.' My present invention discloses rdierent cir?. cuit arrangement which incorporate newfand.

advantageous features. for the solution of such problems; 1.

1927. Serial No. 189,692.

' There are a large number of different types of filter circuits having different attenuation characteristics.

They are all characterized however by the common feature that regardless'of the type of filter there are four ter minals7 two input and two output..

The design of filters to meet specified attenuating and load conditions is complicated and it will suffice tol here record that three of the fundamental elements entering the design, are, the cut off frequencies7 the degree of cut off, and the impedance termination of t-he filter. In general the inductive-elements of the filter increase in size in direct proportion, and the capacitative elements decrease in size in inverse proportion to the magnitude of the resistance component of the loadimpedance which terminates `the filter, viz., which is connected between the output terminals 16--17 of Fig. l.

A load circuit of high resistance Awill thus require a filter having a high ratio of inductance to capacity; while a load circuit of low lresistance will require a filter having al respectively of filters 7, 8, 9, are shown connected in A to the three phase electric power transmission system 16, 17, 18. Viewing the three phase transmission systemk from the points 16u17. 17-18, and 18-16, the circuitsY may possess different input resistive properties, effective at high frequency.. The filters 7, 8, 9, will therefore be designed accordinglyand while having the same cut olf frequencies will have different ratios of inductance to capacity. That is, three complete ilters are required. A

til

Experiment has shown that in a large number of cases the high frequency input resistance of one phase of a polyphase system is considerably higher than the input resistance of the other phases, which may be of the same order of magnitude. This feat-ure permits of a great simplification in filter construction with a resulting reduction in cost. This is made clear by reference to Figs. 3 and 4, wherein it is assumed that the input resistance of phase 16-1S is high compared to those of phases lf3-17, and 17-18- It will be noted that only two filters are employed instead of three as in Fig. l.

Referring particularly to Fig. 8, the input terminals of filter I9 are shown at 2l, 22; the input terminals of filter 20 are 22, 23; the input terminals of combined filter 19-20 are 23-21. Consideringr first the series connected elements, the ratio of inductance to capacity of the first and last meshes of both filters 19 and 20 is For the combined filter 19-20, this ratio is l p1) 201 201 p1 2 T 2 l 401 O1 is four times that of the other two filters. For the intermediate mesh, the ratio for lters 19 and 20, is

and for the combined filter 19-20 is all: 4a 501 O1 or again four times that of the other two filters. Considering` the parallel connected elements of the filter between points 24e-25, 25-26, and 27-28, 28--29 the ratio of inductance to capacity eective is Between points 24;-26 and 27-29, however, the ratio of inductance to capacity is extraordinarily effective in practice. In fact, the filter arrangement of Figs. 2, 3, 4 may properly be called an open A filter as in Figs. 2 and 3 which have closed A input transformers, the presence of secondary 5 in no way affects the design or construction of the filter.

In Fig. 5, is shown the same type of filter operating from'a three wire two phase transformer. Here the change in ratio of inductance to capacity is not so important as very rarely is there any load connected to phases 16-18, only the two phases lG-17. and 17-18 being employed. However, it is not intended to restrict the scope of the invention to three phase systems.

The polyphase filter system of my invention has been found to be very practical in operation in polyphase systems of wired radio broadcasting.

While I have described my invention in certain preferred embodiments, and have illustrated its use with only one type of filter, I desire that it be understood that modifications may be made, other types of uniform or composite filters may be employed, and that no limitations upon my invention are intended other than are imposed by the scope of the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is as K follows l. In a wired radio broadcasting system the combination of a source of three phase highfrequency current, a three phase electric power transmission system, a plurality of filter circuits having common elements interposed in series between said source of three phase high frequency current and said three phase electric power transmission system, each of said plurality of filter circuits being associated with dierent phases of said power transmission system and shunt connected filter circuits including parallel related inductance and capacity elements disposed in each phase, the ratio of inductance to capacity of both the series and shunt connected branches of one of said filter circuits having a larger value than the ratio inductance to capacity of the remaining of said plurality of filter circuits.

2. In a wired radio broadcasting system the combination of a source of polyphase high frequency current, a polyphase electric power transmission system, a plurality of filter circuits` having common elements interposed in series between said source of polyphase high frequency current and said polyphase electric power transmission system, each of said plurality of filter circuits being associated with a different phase of said power transmission system and parallel connected branch impedance circuits disposed in parallel in each phase, the ratio of inductance to capacity of both the series and parallel con- Cil nected branches of one of said filter circuits having a larger value than the ratio inductance to capacity of the remaining of said plurality of filter circuits.

3. In a wired radio broadcasting system comprising a source of three phase high frequency current, a three phase electric power transmission system, a filter circuits interposed between lsaid source of three phase high frequency current and said three phase electric power transmission system, said filter circuit having three input and three out- Vput terminals and comprising independent series and parallel connected composite impedance elements, said filter circuit being so constructed that when viewed from one pair of said three terminals it will have a higher ratio of inductance to capacity elements than when viewed from the two remaining pairs of said terminals.

4. In a wired radio broadcasting system a source of polyphase signaling current having at least three output terminals, an electric power transmission system having at least three conductors, in which the impedance at signaling frequencies between a first and second of said conductors is substantially greater than between either of those conductors and a third conductor, and a i'ilter system connecting said source to said line comprising three series paths, two of which include lumped impedance elements and connect to said first and second conductors respectively and the third of which is of negligible impedance and connects to said third conductor.

5. A system as described in claim 4, further characterized in that shunt impedance elements are connected insaid filter system between said first and third paths, but not between said first and second paths.

In testimony whereof I aiX my signature.

ROBERT D. DUNCAN, JR. 

